That is, for example, to have fullfilled a Sunday Mass requirement, when must the person arrive and when can he leave, under normal circumstances? I understoood that the Gospel reading and final blessing were the parameters in the past.

There are no "minimum requirements." The Church wants you to get to Mass on time and stay for the whole Mass. If just reason unavoidably delays your arrival or compels your departure, there is no sin in that. But to ask "when must the person arrive and when can he leave" implies that attendance at Mass is a burden to be shed as quickly as possible, rather than a privilege and a gift granted by God through his Church.